Vinca require low fertility levels between 100 and 150 ppm N. Low fertility results in chlorotic (yellow) stunted plants with an upright leaf orientation. Optimal substrate pH values range from 5.5 to 5.8. If the substrate pH drifts <5.5, iron (Fe) and manganese can accumulate at toxic levels and observed as lower leaf black spotting. Substrate pH values >6.5 inhibit Fe uptake, leading to symptoms of interveinal chlorosis and bleaching (whitening) on the upper leaves.
Read more at e-GRO (W. Garrett Owen, Josh Henry, Brian E. Whipker)